1. Technical Field
The present invention relates to a projector, and in particular to a projector increasing the number of pixels using an optical method to thereby make it possible to display a high resolution projection image.
2. Related Art
In conjunction with the spread of high-resolution images, the demand for the projector capable of displaying higher-resolution images with higher brightness has been increasing. As a measure for making the display image high-resolution, there can be cited increase in the number of pixels of the light modulation element such as a liquid crystal light valve. However, if the number of pixels is increased without changing the pixel size, the size of the light modulation element increases, and the manufacturing cost rises significantly. In addition thereto, since the size of the optical system treating the image light emitted from the light modulation element is also increased, the significant rise in the manufacturing cost is unavoidable. By contraries, if it is attempted to increase the number of pixels without changing the size of the light modulation element, it is required to reduce the pixel size. However, the miniaturization of the switching elements and the wiring in the light modulation element are difficult and have limitations. Therefore, the aperture ratio is degraded with the reduction of the pixel size, and the light intensity of the image light is lowered, which results in dark display images.
Therefore, there has been proposed a measure for increasing the apparent number of pixels using an optical method without increasing the physical number of pixels of the light modulation element to thereby achieve high-resolution display images (see JP-A-8-29779 (Document 1)). In the display device described in Document 1, there is adopted a configuration of disposing a polarization rotation element and a birefringent optical element on the exit side of the liquid crystal display (LCD) for generating the image light to thereby shift the light path of the image light transmitted through these elements. In Document 1 mentioned above, there is a description that the high-resolution image display becomes possible even with the low-resolution display element by shifting the light path of the image light between the fields consecutive on the time axis and then displaying the images at different positions to thereby double the apparent number of pixels.
Incidentally, the light modulation element (hereinafter referred to as a liquid crystal light modulation element) using the liquid crystal is generally not provided with an image memory for holding the image data pixel by pixel. Therefore, in the liquid crystal light modulation element, there is generally adopted a method in which the image data is sequentially written line by line to finally form a image corresponding to one frame, a so-called line sequential method. In other words, in the display image, the image of the present sub-frame is always rewritten with the image of the succeeding sub-frame line by line. Therefore, in the images formed by the liquid crystal light modulation element, each of the images includes both of the image of the present sub-frame and the image of the newly rewritten successive sub-frame at a certain time point.
In the display device described in Document 1 mentioned above, the polarization plane of the image light emitted from the LCD is rotated throughout the entire image in a lump at predetermined timing field by field using the polarization rotation element. In this case, it is not achievable to synchronize the operation of continuously rewriting the image data line by line by the LCD and the operation of simultaneously shifting the light path of the image light throughout the entire image at certain timing with the polarization rotation element and the birefringent optical element with each other. Therefore, it is not achievable to realize the high resolution throughout the entire area of the image, and the image becomes partially redundant to cause degradation in the image quality.
Further, in Document 1 described above, there is a description that the advantage of increase in the number of pixels can be enhanced by dividing the electrode of the ferroelectric liquid crystal cell constituting the polarization rotation element into two or more (specifically five) line electrodes to make it possible to shift the light path of the image light for each of the areas corresponding respectively to the line electrodes. However, the image light emitted from the LCD is diverging light, and the light beam diameter is significantly expanded when the image light enters the polarization rotation element disposed distant from the LCD. Therefore, even if the division number of the electrode of the polarization rotation element is made equal to the number of lines of the pixels of the LCD, it is not achievable to conform the boundary between the image of the present sub-frame and the image of the subsequent sub-frame to the boundary between the areas having respective polarization planes different from each other. Therefore, it is not achievable to make the image high-resolution throughout the entire area thereof, and therefore, it is not achievable to sufficiently avoid the degradation of the image quality.
It should be noted that in the explanation described above, there is cited the example of the line sequential method, which is a typical writing method of the image data in the liquid crystal light modulation element. However, in some cases, there might be adopted a method of sequentially rewriting the image data pixel by pixel, namely a so-called dot sequential method in the liquid crystal light modulation element. The problem described above is also common to the projectors adopting the dot sequential method.